The need for a new and more versatile personnel inspection system in mass transportation centers has increased in recent years. Traditional inspection systems such as metal detectors and X-ray imaging systems, although capable of near real-time detection, have limitations and adverse effects in the detection of concealed targets. Limitations of metal detectors include the inabilities to (a) provide precise target location, (b) detect plastic concealed weapons, and (c) detect certain metals because of sensitivity variation for various metals. Limitations of X-ray imaging of personnel include radiological health effects. Consequently, millimeter wave holography has been under investigation as an alternative or complementary approach to personnel inspection.
Related holographic art known to the inventors includes the following. A report, EVALUATION OF PASSIVE FAR INFRARED RADIOMETRIC TECHNIQUES FOR DETECTION 0F CONCEALED OBJECTS, DT Hodges et al., Aerospace Report No. ATR-79(7745)-1, Contract No. At-(29-1)789, Sandia Laboratories, Albuquerque, N.Mex., 87115, March 1979, p. 41, discloses apparatus and a process for far infrared detection of concealed objects. U.S. Pat. No. 4,841,489 to Ozaki et al. discloses a method for imaging an object or substance by ultrasonic or electromagnetic waves based on a synthetic aperture method capable of economizing memory capacity, achieving real-time base image reproduction, and obtaining a high quality image.
Application of holography to the problem of personnel surveillance has been limited because of the inability to either (a) produce an image of sufficient resolution, or (b) produce an image in near real-time, or (c) a combination of both. It is recognized that use of millimeter wave electromagnetic radiation is not a physiological health hazard and such radiation penetrates certain materials, including but not limited to clothing.
Prior work as reported by NH Farhat, HIGH RESOLUTION MICROWAVE HOLOGRAPHY AND THE IMAGING OF REMOTE MOVING OBJECTS, Optical Engineering, Sep.-Oct. 1975, Vol. 15, No. 5, pp. 499-505, utilized millimeter wave holography in working toward surveillance systems. However, Farhat did not obtain high resolution because he used f-numbers greater than 1.0. Moreover, he could not achieve near real-time imaging because he used an optical reconstruction technique.
Hildebrand and Brenden, AN INTRODUCTION TO ACOUSTICAL HOLOGRAPHY, 1972, Plenum Press, New York, N.Y, demonstrated excellent resolution with acoustical holography using optical reconstruction and a low f-number. However, it is recognized that acoustical holography is impractical for personnel surveillance because of the coupling fluid required between the acoustic transmit/receive element and the target. Moreover, Hildebrand and Brenden used optical reconstruction that cannot achieve near real-time imaging.
Both Farhat and Hildebrand et al. used optical reconstruction to produce images with their holograms. Another reconstruction technique is digital reconstruction wherein the signal reflected from the target in the form of acoustic or electromagnetic radiation is converted into a digital electronic signal that is mathematically converted into information that is useful for producing an image of the target. Even using digital reconstruction techniques, however, high resolution is not always obtainable because the reconstruction techniques inherently limit image resolution.
There are standard digital reconstruction techniques that have been used for processing millimeter wave data. For example, a widely used method is to apply a fast Fourier transform to the Kirchoff diffraction integral. However, this method uses a finite sum written in the small angle (Fresnel) approximation that limits the relationship between aperture size and distance to the target, inherently limiting the f-number to be at least 6, and thereby limiting resolution of holographic image reconstruction.
A digital reconstruction method that overcomes this limitation is reported by AL Boyer et al., RECONSTRUCTION OF ULTRASONIC IMAGES BY BACKWARD PROPAGATION, Reconstruction of Holographic Images, Chapter 18, July 1970. The so-called angular spectrum backward wave propagation method does not use the Fresnel approximation and can therefore be used for low f-number reconstruction. However, the angular spectrum method has not been widely used for holographic imaging because it is generally believed that the recording plane must be very stable and flat within a small fraction of the wavelength. A thesis by HD Collins, June 1970, demonstrates that the recording plane need not be flat within a wavelength. Another reason that the angular spectrum method has not been widely used is that it is computationally intensive.
A two-dimensional image reconstruction algorithm is implemented in U.S. Pat. No. 5,170,170 issued Dec. 8, 1992, to Soumekh. Soumekh derived a wideband SAR (synthetic aperture radar) imaging algorithm which reconstructs data from a linear aperture into a two-dimensional image. This technique, as with all SAR techniques, is not well suited to personnel surveillance imaging because it is limited to use with a linear aperture, rather than a two-dimensional aperture.
The second fundamental limitation preventing wide use of millimeter wave holography for personnel surveillance is the amount of time required to scan a target. Boyer et al. acoustic target scanning required 50 minutes to obtain 300 samples/second in 256 scan lines. Boyer et al. restricted scan rates to obtain highest spatial frequency. Use of a single antenna element moved from position to position for 65,000 positions across an aperture has been accomplished in 5 minutes. To be useful in surveillance, it is necessary to perform a scan within several seconds and preferably in one second or less.
In holography, fast scans with high resolution are difficult to achieve. Resolution is highest when the millimeter wave signal is transmitted and received from the same antenna element. As previously indicated, moving a single element to hundreds of positions is physically limited to scan times on the order of minutes. Use of separate transmit and receive arrays severely limits the resolution of the reconstructed image. Examples of scanning systems include various arrangements of antenna types and arrays.
Antenna arrays have been used as reported in Tricoles et al., MICROWAVE HOLOGRAPHY: APPLICATIONS AND TECHNIQUES, Proceedings of the IEEE, Vol. 65, No. 1, Jan. 1977. In FIGS. 4 and 5 of Tricoles et al., arrays are shown wherein antenna element spacing is much greater than a wavelength of the microwaves and there are separate transmit and receive arrays.
Larson et al., MICROWAVE HOLOGRAM RADAR IMAGERY, February 1972, show a far-field microwave holographic imaging system having a single transmit horn antenna and a separate receiver array of 100 elements spaced slightly more than 1/2 wavelength apart. FIG. 11 of Larson et al. shows an image of a Jeep made with this system. The image resolution is coarse, 15 cm (1/2 foot) by 15 cm (1/2 foot).
The current state-of-the-art in millimeter-wave imaging systems may be summarized by two fundamentally different techniques. The first technique uses a focal-plane two-dimensional array of millimeter-wave detectors placed behind a large lens as described in U.S. Pat. No. 5,073,782 issued Dec. 17, 1991, to Huguenin et al. Huguenin et al. detect either passive energy emitted by the target or active energy emitted by millimeter-wave illuminators. Advantages of Huguenin et al. include: possible real-time operation, relatively compactness, and operation analogous to an optical camera. Disadvantages include: relatively low resolution due to the high optical f-number of a practical configuration, small aperture (lens size is limited by practical constraints), and limited field of view.
The second technique uses a holographic linear array of sequentially-switched transmitter-receivers scanned quickly over a large aperture to actively illuminate the target as described in U.S. Pat. No. 5,455,590 issued Mar. 14, 1994 to Collins et al. Collins et al. use a single frequency that is coherent, which means the phase of the returned signal is recorded as well as the amplitude. The coherent data is reconstructed in a computer to form a focused image of a target without the need for a lens. Advantages of this technique include: near real-time operation, very high-resolution (due to low optical f-number), computer reconstruction allows focusing at any depth, and large aperture (full body field of view). The primary disadvantage of this system/technique is that the close-range, large aperture operation causes the depth of focus to be very short. Therefore, the image of a target with significant depth, such as the human body cannot be reconstructed in complete focus. A further limitation of Collins et al. is interference from a cover that is placed between the person to be imaged and the transceiver system. The cover vibrates thereby producing an interference in the signals between the person and the transceiver system. While it is possible to subtract out some of the interference, it is not possible to remove it completely because the cover does not always vibrate in the same manner. Hence, resolution is limited to the extent of remaining cover interference.
Thus, there is a need for a holographic image reconstruction method and apparatus that can provide high resolution with fast scanning and fast reconstruction algorithm and that has an expanded depth of field to accomplish near real-time imaging that is needed for personnel surveillance.